For use in areas where ambient noise levels are quite high, or where extraneous sounds would tend to become confused with principal voices or music, it is desirable to provide a microphone which can be aimed at a chosen source of sound with its back to unwanted noise. Microphones having a cardioid pattern of response are well known for use under these conditions, it being an object of the present invention to provide an improved microphone of this type. More particularly, the microphone according to the present invention embodies certain features and principles of directional microphones described in applicant Bauer's U.S. Pat. No. 2,237,298 and continuations-in-part thereof which issued as U.S. Pat. Nos. 2,305,596, 2,305,597 and 2,305,598, and in an article entitled "A Review of Cardioid Type Uni-Directional Microphones" appearing in the January 1940 issue of The Journal of the Acoustical Society of America, Volume 11, Page 296. Some of the material presented in these references is repeated here in summary form as background to an understanding of the present invention.
The directional characteristics of microphones are commonly described by a directional sensitivity, or polar, pattern of the kind illustrated in FIG. 1, in which the circle 10 depicts that a microphone 11 located at the center thereof has equal sensitivity, in terms of the output voltage, E, produced at the microphone terminals 12 and 13, to a plane progressive sound wave of r.m.s. sound pressure, p, regardless of the directional angle .theta. from which the sound wave .theta. impinges upon the microphone in the plane of the graph. Since microphones usually are symmetrical about an axis 0.degree.-180.degree. through the length of the microphone body, the circle 10 can also be regarded as the circumference of an imaginary sphere surrounding the microphone; that is, that the microphone is equally sensitive from all directions in space, or "omnidirectional." Assuming that the radius of the circle 10 represents the reference sensitivity of the microphone in terms of the signal voltage produced by sound pressure at the microphone, then half the radius would represent a 50 percent drop in sensitivity, corresponding to a level change of 20log0.5 = -6db, a quarter of the radius would represent a drop in sensitivity of 75 percent, corresponding to a level change of 20log0.25 = -12db, etc. In other words, the polar pattern of FIG. 1 is based on a linear voltage-pressure relationship along the radius vector; thus, if the sensitivity of an omnidirectional microphone is designated S, then in terms of the angle arrival .theta. in a plane through the axis of symmetry, S is defined by the expression: EQU S = 1(.theta.) (1)
This equation simply shows that the value of S as a function of the angle .theta. is unity for all directions of sound arrival.
An important family of directional characteristics based on the so-called limacon family of patterns is characterized by the equation: EQU S = (1-k) + kcos.theta. (2)
which when k = 0 yields the omnidirectional pattern expressed by Eq. (1). For a value of k = 1/2, Eq. (2) becomes S=0.5 + 0.5cos.theta., and the dashed-line pattern 14 in FIG. 1 is produced, this being the familiar "cardioid" pattern. When k has a value of 0.75, Eq. (2) becomes S=0.25 + 0.75cos.theta., and the pattern shown in dash-dot line is produced, with its major lobe 15a directed toward the front and with a smaller lobe 15b directed toward the back of the microphone. Finally, when k has a value of 1.0, a "cosine pattern" is produced which is, in effect, bi-directional because it has lobes of equal sensitivity toward the front and the back as depicted by the dotted line circles 16a and 16b, respectively.
The nature of the present invention and the applicability thereto of the above theoretical discussion will be better understood from the following description of two different types of directional microphones disclosed in the aforementioned patents. FIG. 2 shows in stylized cross-section a microphone mechanism having a curvalinear coneiform diaphragm 20, the outer surface of which is exposed to the oncoming sound wave designated p.sub.1. Normally this mechanism is mounted in a suitable foraminous case (not shown) for protection and ease of handling. Assuming that the wave arrives from the head-on or 0.degree. direction as indicated, it must travel, because of the presence of the microphone case 22, an additional equivalent distance d before it arrives to the passages 24 in the back of the case. At this point the sound wave has a pressure p.sub.2 of the same magnitude as p.sub.1, but differs from it in phase by an angle .phi. = 2.pi.fd/c. If the wave arrives at an angle other than 0.degree., the effective distance becomes d.sub..theta., which differs from d by cos.theta., as depicted by the double arrow 25, which represents the equivalent distance d projected upon the axis at 0.degree.. Thus, the phase angle between p.sub.1 and p.sub.2 becomes (2.pi.fd/c) cos.theta., this factor being important to the explanation of the directional performance of the microphone.
The sound pressure p.sub.2 causes an acoustical flow through apertures 24, which are usually covered by a fabric 26, causing compression of the volume of air within the cavity 28 defined by the inner surface of the diaphragm and the microphone case and development of a sound pressure p.sub.3 therein. The pressure differential between p.sub.1 and p.sub.3 acting upon the diaphragm causes it to move, and via a connecting rod 30 to actuate a transducer 32 which generates an output voltage E at the transducer terminals 34 and 36.
Analyzing the acoustical elements of the microphone in terms of their electrical network equivalents, the mass of air in apertures 24 may be considered to approximate an inductance L.sub.A, the flow resistance of the fabric 26 may be considered as a resistance R.sub.A, and the volume of air within the cavity 28 may be considered as a capacitance C.sub.A. When these elements are properly selected relative to the distance d as taught in the aforementioned references, the pressure p.sub.3 at the inner surface of the diaphragm can be made substantially equal to the pressure p.sub.2 but displaced from it in phase by a preselected phase angle .phi..sub.1. By designing the microphone so that the phase angle .phi..sub.1 has a predetermined relationship with the phase angle .phi. the microphone can be made to have any desired sensitivity pattern within the range encompassed by Eq. (2). For example, when .phi. and .phi..sub.1 are equal in magnitude the microphone has a cardioid polar pattern, as will be seen by examination of the phasor diagrams of FIGS. 2A and 2B. In FIG. 2A, which shows the sound pressure relationships corresponding to a 0.degree. incidence of the wound wave, the phase angle .phi.=2.pi.fd/c is the same as that produced by the phase shift network, .phi..sub.1, the latter angle being selected to be equal to 2.pi.fd/c. The pressure difference across the diaphragm may be thought of as the length of a phasor connecting the ends of the arrows p.sub.1 and p.sub.3 as the direction of sound incidence changes as the sound source moves around the microphone, the phase angle .phi. is modified by the change of the equivalent distance d.sub..theta. by the factor cos.theta., and the pressure phasor p.sub.1 may be thought of as moving along the dashed-line from point g (for 0.degree. incidence) down to point h (for 90.degree. incidence) and finally down to point i (for 180.degree. incidence). The latter situation is portrayed by the phasor diagram in FIG. 2B which shows that for rear incidence the two phasors p.sub.1 and p.sub.3 are coincident, there being, therefore, no pressure difference to actuate the diaphragm with the consequence that the output of the microphone is zero. The net sensitivity, as a function of the azimuth angle .theta. is clearly related to equation S=0.5 + 5cos.theta., which defines the cardioid pattern described previously.
The aforementioned patents teach that acoustical networks for giving various type of transducers desired directional properties can take on a number of different forms and also describes ways of proportioning such networks, and the underlying theory need not be repeated here. Of particular significance to the present invention is that when a coneiform diaphragm is used to drive a transducer via a slim drive rod, the area of the diaphragm exposed to the interior cavity of the microphone is very nearly the same as the area exposed to the sound field, thereby ensuring that the forces across the diaphragm have very nearly the same relationship as the pressures whereby the desired limacon pattern is very nearly followed.
Because a moving coil or dynamic microphone is more rugged, and its impedance lower than the microphone just described, it would be desirable to incorporate the advantages of a coneiform diaphragm into a transducer of the moving coil type. However, this poses the design and structural problems exemplified by the moving coil microphone shown in stylized cross-section in FIG. 3. The diaphragm 40 of this known type of microphone consists of a dome 42 and a flexible rim 44, and has a circular coil of wire 46 attached at the juncture between the rim and the dome, the terminal leads 48 and 50 thereof which collect the voltage generated in the coil being brought out to the exterior of the microphone. The coil is immersed in a strong magnetic field produced in the gap between an inner pole-piece 52 and an outer pole-piece 54 produced by a magnet 56 and the surrounding return path member 58. In the conventional dynamic microphone, the dome 42 (also known as a piston) constitutes the most pertinent active area of the diaphragm and therefore is the principal contributor to the acousto-mechanical function of the transducer. The rim portion 44a provides a seal to the microphone case and flexibility, but because it rests upon the edge of the case, part of the force of the incident sound pressure is borne by the case and is not transmitted to the moving coil. Thus, the rim portion 44 has appreciably less influence upon the performance of the microphone than the dome portion, the main concern of the designer being to keep the rim axially flexible and tangentially stiff (to avoid spurious resonances), which is usually accomplished with corrugations and/or other stiffening devices.
The above-outlined advantage of having substantially equal inner and outer surfaces in the active region of the diaphragm would suggest that the dome area of the diaphragm in a moving coil microphone be made as large as possible, that spaced entrance ducts 60, depicted by the dashed lines, be provided around the rim of the diaphragm, and that these openings be covered with a fabric 62 to introduce a suitable acoustical flow resistance. This design approach has the shortcoming, however, that the sound pressure p.sub.3 developed at the exit of ducts 60 would act upon the backside of the relatively ineffective rim area 44 of the diaphragm, and the acoustical flow would have to travel the added path between the moving coil and the magnetic structure indicated by the arrow 64 before generating the sound pressure p.sub.4 within the cavity behind the dome area, the only significant active area in this type of microphone. These complications make it difficult to design a phase-shift network having appropriate interaction with the dome activity to achieve the desired directional sensitivity. Although the aforementioned patents suggest that the rim area be eliminated and the diaphragm be suspended on flexible metal tabs to allow main entry into the dome area to take place through a slit between the moving coil and the inner pole-piece of the magnetic structure, and other more contemporary designs attempt to circumvent the problem by providing passages between the moving coil and the diaphragm, all of these designs are complicated, are difficult to construct and suffer from a lack of mechanical strength and stability.
Accordingly, it is the primary object of the present invention to provide a directional microphone of the moving coil type having a relatively larger active diaphragm area than is exhibited by the diaphragms of known microphones of this type, and which is relatively simple to construct.